I am thinking of two people.
The first person is a member of my parish. This is where I meet him. He is a nice guy. I see him at Mass on Sunday. Later, I realize that he too is a mathematician and works for an employer similar to mine.
The second person is a colleague. I have met him at work. Like others in our field, I respect his technical expertise. Later, I find out that he too is Catholic. Occasionally, I go to Mass at his parish on my way to work and see him there.
In a sense, I may think of the first man as "a Catholic who happens to be a mathematician" and of the second man as "a mathematician who happens to be Catholic."
Is there a difference?
For me as a fellow Catholic, there may be no difference. I recognize the necessity for a Catholic to integrate his faith and his work: to be a "good Catholic" on the job and to do well at a job as an expression of the values of faith. Both of these men are valuable role models to me.
But if I was not Catholic, would there be a difference in the potential for these men to be an "undercover missionary" to me?